This invention relates to a method for electrolytic treatment which electrochemically roughens the surface of an aluminum web for the support used for a presensitized printing plate and the like.
Generally, an aluminum web is utilized as the support for a presensitized printing plate and the surface thereof is usually roughened in order to improve adherence between the aluminum web and the photosensitive layer provided thereon and to retain water utilized at printing.
Conventionally, there was a process as disclosed in U.S. Pat. No. 5,082,537 (corresponding to Japanese Patent KOKAI No. 2-298300) as the roughening process. In the process for roughening, a substrate is conveyed through an aqueous electrolytic bath having a plurality of electrodes and a three-phase or alternating current is applied to the electrodes, wherein the frequency of said three-phase or alternating current is higher than a line frequency of 50 Hz to 60 Hz preferably between about 50 to 300 Hz and the frequency is selected at a value that is related directly to the rate of conveyance of the substrate through the electrolytic bath.
However, in the above mentioned process disclosed in U.S. Pat. No. 5,082,537, a uniformly roughened surface can not be necessarily obtained. That is, since a degree of so-called electrical cross-strokes (a nonuniformity generated in accordance with alternative currents and movement of a material to be treated, when an electrolytic roughening treatment is conducted by supplying alternating current at a very high working speed) is decided by the travelling speed and the frequency of a source, when the line speed is changed, the frequency of a source should be changed according to the change of the line speed in order to obtain the same cross strokes. However, since the degree of a roughened surface is changed according to change of the frequency, a uniformly roughened surface cannot be obtained.
Besides, in the above mentioned conventional method, the contrast of a cross stroke is high and conspicuous.
Moreover, in the above mentioned electrolytic treatment using an alternating electrolysis, there are two ways to increase the amount of electrolytic treatment, i.e. one is a method of increasing the length of electrolytic treatment and the other is a method of increasing current density in the former method, it is necessary to provide not less than two electrolytic treatment baths, and consequently, electric sources are provided respectively for each electrolytic treatment bath.
However, in the above mentioned method for electrolytic treatment, cross-strokes significantly varied periodically.